Begin by measuring the box width(A) of the trailer. Technically the overhang(B) is measured from the center of the spring hanger, but for the sake of simplifying this process we will measure from the outside edge of the box. Below is a table giving acceptable limits of overhang (I have subtracted the difference of the overhang out of the table) for different sizes and styles of axles. Add box width(A) to the (minimum and maximum individually to get a range) overhang(B) and you get the (acceptable range of) axle hub face(C) for the box size of your trailer. Be sure to also take into account fenders and offset of wheel/tire combos when narrowing it down to the hub face that fits you needs. It is smart to figure approximately 1.5"+ clearance between your tires and trailer box and/or fender backs to allow for margin of error.
Tubular Straight (TS) / Tubular Drop (TD)
 Axle Type Minimum Maximum
TS2000 9" 17"
TS3500 11" 17"
TD3500 13" 17"
TS5200 12" 16"
TD5200 14" 18"
TS6000 12" 16"
TD6000 14" 18"
TS7000 12" 18"
TD7000 14" 19"
TS8000 15" 19"
TD8000 16" 19"
9000 25" 32"
12000 25" 32"
Torsion Axles (EQ)
 Axle Type Minimum Maximum
EQ2000 12.5" 22"
EQ3500 12.5" 24"
EQ6000 14.5" 24"
EQ7000 14.5" 28"
EQ8000 17.5" 43"